1. Field of the Invention
The present invention relates to an improved method of measuring stress in materials. In particular, the present invention is a method for measuring residual stress in a variety of materials, including metals, using a method which combines laser interferometry and laser annealing in conjunction with a liquid temperature indicating coating.
2. Discussion of Background
Materials used in construction that are made of metals and metal alloys are prone to a phenomenon known as stress corrosion cracking. In this phenomenon, cracks appear in areas of tensile stress, such as welded joints, as a result of the migration of chloride atoms to grain boundaries in the material. Tensile stresses are induced in manufacturing operations such as bending, heat treating, grinding and welding. The presence of stress in material is one factor in the cause of stress corrosion cracking, but other factors are also important. Two additional factors are the specific metallurgical makeup of the material and the environment of use.
Residual stresses add to the load applied to a part used in construction. If the structural design is not sufficiently conservative, the part can fail from the combination of the load and material stresses. By having a method for accurately measuring the residual stresses in individual parts, a designer can have a better understanding of the total loads on them. With that understanding, the designer can predict failure with greater certainty and design to avoid failure or to relieve the stresses.
Stress measurement at the surface of a metal object is especially important because most failures begin there. Surface stress will result in cracks that propagate more rapidly, then tend to pull the material apart. Welding in particular imparts surface stresses because it imposes sharp temperature gradients between the exterior and the interior regions of a material, resulting in plastic deformation of the exterior and elastic deformation of the interior of the material.
Residual stresses are currently measured in a number of different ways. Both destructive and non-destructive methods are available to determine residual stress in the object of interest. Destructive methods include those based on drilled specimens or chemical etching. Nondestructive methods include acoustic, x-ray, magnetic and optical methods.
Several residual stress measurement methods employ an annealing process to provide stress relief due to localized melting. Of particular interest is the non-destructive optical method disclosed by Pechersky in U.S. Pat. No. 5,432,595, incorporated herein by reference, involving the use of a combination of electronic speckle pattern interferometry (ESPI) (hereinafter laser interferometry) and laser annealing. Methods involving the use of an annealing step require that the amount of heating be carefully controlled and that the peak temperature of the spot be known precisely.
Nothing in prior patents or methods teaches the use of temperature indicating coatings in residual stress measurement. Temperature indicating coatings have been found to provide significant advantages in temperature control and detection to those stress measurement techniques which rely on an annealing process. The use of temperature indicating coatings have proved to have particular advantage when used to measure stresses in metal.